This invention relates to a method of and apparatus for controlling the weight of samples dissolved or suspended in a liquid while they are evaporating in a vacuum. It is particularly applicable to samples in centrifugal evaporators.
Samples to be evaporated in centrifugal evaporators are usually held in glass or plastic tubes or, sometimes, in a large number of small wells in plastic blocks. The sample holders are mounted upon a rotating assembly and spun at relatively high speed so that a considerable centrifugal force is applied to them in an outward direction, which forces the liquid to the lower part of the sample tubes and prevents any frothing or spitting of the liquid out of the sample tubes when a vacuum is applied. The spinning samples are held in a vacuum-tight chamber (referred to herein as a xe2x80x9cchamberxe2x80x9d) which is connected to a vacuum pumping device.
Centrifugal evaporators of this type are well known and many types are available commercially. One problem from which such evaporators suffer, is that it is very difficult to obtain a desired continuous read-out of the weight of the sample in the holders as the liquid is being evaporated, since the holders are being spun at a high speed, typically at about 1400 r.p.m. The possibility has been considered of continuously weighing the whole evaporator during spinning. However, this involves measuring a total weight of the order of 50 kg to an accuracy of about 1 gm, which is a very demanding task.
Another problem arises when evaporation needs to take place simultaneously for different solvents, or solvent mixtures of differing compositions, in which the samples are dissolved or suspended. In this situation those samples which are held in the more volatile solvents or mixtures will evaporate faster than the ones held in the less volatile solvents, and this can lead to an excessive imbalance in the rotating assembly, and consequent unwanted vibrations. This would also mitigate against the possibility of weighing the whole evaporator.
In most centrifugal evaporator machines such unwanted vibrations are arranged to trip an out-of-balance sensor to thereby stop the machine, but in machines without a sensor the vibrations can cause damage to the machine and even to the samples. Sometimes the vibration problem can be overcome by careful loading of the evaporator, or by stopping the process from time to time and rebalancing the load by adding liquid to empty samples or by rearranging the samples in the rotating assembly. Both these methods are tedious and time consuming.
It is an object of the present invention to enable the weight of a sample in a centrifugal evaporator to be continuously and accurately measured during evaporation.
It is another object of the invention to enable the operation of a centrifugal evaporator to continue despite a considerable imbalance of forces.
According to one aspect of the present invention, a method of evaporating a liquid sample contained in a sample holder which is mounted within a chamber and rotated by a rotor therein during the evaporation so that centrifugal force is exerted on the contents of the sample holder during the process whilst a pressure below atmospheric is maintained in the chamber in manner known per se, so as to leave as a residue any solid material dissolved or otherwise mixed in the liquid forming the sample, characterised by: mounting a transducer to monitor the force acting on the sample holder relative to the rotor when rotating at a given speed and obtaining a force signal therefrom, supplying the force signal to a computer means, programming the computer means to compute a value equivalent to the centrifugal force exerted on the sample holder due to rotation of the rotor at said given speed, further programming the computer means to compute a weight value from the force signal using the computed centrifugal force, and further programming the computer means to generate a control signal for controlling the evaporation process in dependence on the computed weight value.
In some circumstances the rotor may be rotating at constant speed, so that the weight value can be computed for that particular speed.
Alternatively, however, the method may further comprise the steps of mounting a second transducer to monitor the speed of rotation of the rotor, obtaining a speed signal therefrom, and supplying the speed signal to the computing means for computing said weight value.
Preferably the computing means is adapted to rotate with the rotor.
Preferably the computing means is programmed to convert the output of the sensor into a form suitable for transmission to an external receiver.
Preferably the computing means converts the transducer signals into digital signals by which a carrier signal is modulated to effect the said transmission.
In general the transducer signals are produced continuously and the weight and centrifugal force factor values are continuously computed therefrom.
Conveniently the computing means has stored therein a value equivalent to the weight of the sample holder, and is further programmed to compute a value equivalent to the weight of the contents of the holder by deducting from the computed weight value a value equivalent to the known weight of the sample holder.
Preferably the computer means computes the rate of change of the computed weight value.
Preferably the method includes the step of heating the sample during rotation in the chamber to increase the rate of evaporation.
Preferably the method includes the step of controlling the supply of heat to the sample in dependence on the computed weight value, preferably in dependence on the computed rate of change of weight value.
In general, the supply of heat will be reduced as the rate of change of weight with time starts to decline, and the evaporation process is terminated when the rate of change drops to zero, indicating that the sample is dry.
The invention also lies in apparatus for evaporating a sample comprised of solid material dissolved or suspended in a liquid, comprising a vacuum chamber, a rotor therein, drive means for rotating the rotor relative to the chamber, a sample holder for containing the sample connected to the rotor, transducer means associated with the sample holder and the rotor for generating a force signal indicative of the centrifugal force acting on the sample holder as it is rotated at a given speed, and means for transmitting transducer signals to computing means programmed to convert the signal at any instant to a computer value proportional to weight, the computing means being further programmed to generate a process control signal for controlling the evaporation process in the chamber.
The force transducer may be a load cell, or a strain gauge, or where the sample holder is movable relative to the rotor, the force transducer may be a position sensor adapted to produce a signal indicating the position of the sample holder relative to the rotor, as determined by the instantaneous centrifugal force acting on the sample holder, causing it to move relative to the rotor.
Where the movement is permitted, preferably resilient means is provided which resists the movement of the sample holder relative to the rotor.
A plurality of sample holders may be mounted on the rotor and a force transducer is provided for selected ones, or all of, the holders.
The weight of the sample can be calculated from a force value by taking account of the centrifugal force and deducting the known weight of the holder, but an equally useful measurement is that of the rate of change of weight. This is a direct measurement of mass flow rate and can be used to monitor the progress of the evaporation and to reduce the heat when the rate starts to decline, when the samples are nearly dry and to shut the system down when it drops to 0 indicating that the samples are dry.
According to another aspect of the invention in the processing of samples in a centrifugal evaporator in which the samples are dissolved or suspended in liquids of differing volatility, any imbalance caused during spinning of the rotor and resulting in unwanted vibration is at least partially compensated for by associating with the rotor an automatic balancing unit.
The invention therefore also lies in comprising a vacuum chamber, a rotor mounted therein for rotation in use about a generally vertical axis, a drive means for rotating the rotor, at least two sample holders mounted on the rotor, each sample holder being in use about a generally horizontal axis in a radial manner relative to the axis of rotation, a bearing raceway incorporating a plurality of ball bearings which do not fully occupy the circumferential extent of the raceway and which in rotation are automatically distributed around the raceway to counteract any imbalance forces experienced by the raceway, the bearing raceway being mounted to the rotor or a spindle driving the rotor, thereby to reduce any imbalance caused during the spinning of the rotor as result of differential evaporation of liquids from the sample holder.
The ball bearings may be formed from a high density material such as Tungsten or depleted Uranium.
The invention also lies in a method of measuring the weight of a liquid sample in a sample holder attached to a rotor in a vacuum chamber of an evaporating centrifuge, comprising the steps of mounting a transducer to monitor the force acting on the sample holder relative to the rotor during rotation, supplying a force signal to a computing means having stored therein a stored weight value corresponding to the empty weight of the sample holder, the computing means being programmed to convert the force signal to a weight value for a given speed of rotation of the rotor, the computing means being further programmed to deduct from the computed weight value said stored weight value.
The method may further comprise the steps of monitoring the speed of rotation of the rotor, and supplying a speed signal to the computing means for computing said weight signal.
The weight measuring method may be enhanced by mounting to the rotating parts of the apparatus an automatic balancing aid, to counteract any out of balance force arising from differential evaporation of samples.
Only limited space is available within apparatus as described herein for laboratory use and the like, and therefore it is to advantage to use rolling elements constructed from dense materials such as Tungsten or depleted Uranium, since this allows the overall size of the raceway to be reduced both in depth and diameter, due to the increased mass of the rolling elements obtained by using high density materials therefore.